We are seeking to renew a multi-year conference grant which has established a forum for researchers to pursue collaborative studies of the molecular genetics of attention deficit hyperactivity disorder (ADHD). The original application was conceived in response to a recent call from the NIMH for researchers to establish mechanisms for pooling molecular genetic and clinical data in a manner that would facilitate the detection of genes predisposing to psychiatric disorders. ADHD is a common disorder of childhood associated with school failure, psychiatric comorbidity and psychosocial disability. Because family and twin studies suggest that ADHD has a substantial genetic component, several research groups have now begun molecular genetic studies of the disorder. These studies have already produced several replicated findings implicating the DRD4, DAT, DRD5, SNAP-25, and 5HT1B genes in the etiology of ADHD. Although these results are encouraging, several considerations suggest that fully clarifying the genetic architecture of ADHD will require large samples and collaborative efforts. The field's experience with studies of schizophrenia, autism, Tourette's syndrome and bipolar disorder show that the sample sizes collected by individual investigators are big enough to provide evidence for linkage but are not sufficient to clone disease genes. This is consistent with statistical considerations regarding the power to locate genes and the likely genetic complexity of psychiatric disorders. These considerations suggest that coordinated collaboration among investigators is needed to share findings and pool samples. Despite this need, such collaboration is difficult. Many investigators are concerned that large collaborative studies will dilute the scientific impact of their work and will make it difficult for junior investigators to establish independent reputations. Moreover, when collaborations are considered, they frequently face hurdles that cannot be surmounted. For example, clinical traditions at each site often clash regarding what diagnostic instruments are appropriate for use. This leads to the creation of data sets that are not easily combined with one another. Thus, the aim of continuing the currently funded conference grant is to overcome the hurdles to collaboration by continuing yearly conferences among investigators studying the genetics of ADHD.